The need to test foodstuffs in an industrial context has increased over the last twenty years for a number of reasons. Firstly as the world becomes more litigious, manufacturers need to ensure that no foreign bodies are included in with their products: a single piece of glass could potentially ruin a company.
Secondly, as manufacturers mass-produce their products, there is a need to ensure that the quantity of food within a container is exact. For example an extra 3 cc within each soft-drink bottle over a production run of hundreds of thousands of units per week will cost the manufacturer thousands of pounds, which can be saved by using a more exact on-line measuring system in the packing/bottling plant.
Thirdly, processed meat and fish products such as beef-burgers, fish-fingers and ‘chicken nuggets’ will inevitably contain a small number of bones, as a consequence of the processing not being 100% perfect. This is highly undesirable, as finding a bone in one of these products is unpleasant to the consumer, especially if he or she is a child. This will tarnish the reputation of the manufacturer, and will reduce the repeat-buying of the product. A method of checking for bones in the ‘de-boned’ product is therefore highly desirable.
Finally, there are an assortment of material properties of the food material that might be desirable to measure. These might include measuring the homogeneity of the material (for example the homogeneity of strawberry jam is determined by the number of strawberries in it), or the velocity profile of the material, which can be used to measure the temperature distribution within a container. It is also desirable to be able to test the characteristics of the packaging material itself, since this could also affect the product shelf-life.
There has been considerable activity in this general area: much research has been carried out on the use of physical measurements to determine the properties of food. This can be to determine the physical properties of the foodstuff (eg. in emulsions, powders and other forms), or to detect foreign objects, surface defects and food contamination. In particular, there has been recent interest in using ultrasound to investigate the content of food products. One reason for this is that any change in the acoustic property of the test medium could then be related to changes in the food product.
Ultrasound has the ability to differentiate between both the propagation velocity within various media, and the differences in acoustic impedance between different regions within a given volume. Thus, using the usual contact or immersion techniques, ultrasound can be used to measure the moisture content of the food products and for liquid level measurement. However, these techniques require a coupling medium between the test sample and the transducer surface and this can be a considerable drawback. In certain cases, the need to use a couplant makes testing difficult, for instance if contamination of the food or container has to be avoided. For these reasons, X-rays have been used to detect anomalies or foreign objects present in food, usually in through-transmission. This is undesirable for several reasons, including cost and operator-safety concerns. Other techniques such as Magnetic Resonance Imaging (MRI) can be used to study the temperature distribution in food samples, although this is an expensive and complicated method.